1. Field of the Invention
The present invention relates to a network discovery method and apparatus for a cellular mobile radio network, and in particular to a network discovery method and apparatus for identifying frequencies and codes allocated to cells within such a network.
2. Description of Related Art
The present invention has particular application to cellular mobile radio systems operating according to the GSM, DCS1800, or the PCS1900 standards. Systems operating according to these standards, including derivatives thereof, will hereinafter be referred to as "GSM-type" systems. It should, however, be noted that the present invention is not restricted to a GSM-type system.
Cellular mobile radio systems, such as GSM-type systems, comprise a fixed part having switching elements and radio elements providing radio coverage in a plurality of cells, and mobile stations for communicating with the fixed part of the system. Cells are often logically grouped into location areas. Within each cell, radio coverage is provided by one or more radio elements, which, in the case of GSM-type systems, comprise elementary transceivers termed TRXs (Transmitter Receiver). Radio elements are grouped within Base Transceiver Stations (BTS) which communicate with Base Station Controllers (BSC). The interface between a BTS and BSC, within a GSM-type system, is referred to as the Abis interface, and individual links (generally comprising two physical channels) on this interface are termed Abis links. BSCs communicate with mobile switching centres (MSC) via what is termed the A interface.
Cellular mobile radio systems are based on the concept of frequency re-use, that is the use of the same frequency for communication with a mobile station by several cells which are far enough away from each other that interference is avoided. Frequency re-use greatly increases the capacity of the mobile radio system but also leads to greater complexity. For example, a means must be found for ensuring that a mobile station within any particular cell will be able to establish communication with the fixed part of the network on an appropriate frequency. GSM-type systems employ a frequency, known as the beacon frequency, which is broadcast continuously (ie. in every burst period) within each cell. This frequency is utilised by all common channels broadcast within the cell and any activated mobile station whether in idle mode (ie not in use by the user), or in active mode will listen to this beacon frequency.
Due to the scarcity of radio frequencies available for mobile communications, there are generally insufficient frequencies available to a network operator to ensure that a mobile station will not receive two identical beacon frequencies from different cells. To enable the mobile station to distinguish between cells in these circumstances a code, the BSIC (Base Station Identity Code), is broadcast frequently within each cell.
The beacon frequency and code for a particular cell are allocated by the network operator when planning frequency use within the network.
In order to monitor, and optimise, the performance of a cellular mobile radio network it may be necessary to determine the allocated frequency and code for particular cells within the network. Although this information may be derived from switching elements, such as BSCs and their related databases, within the fixed part of the network, there are problems in doing so. Firstly, although many aspects of GSM-type networks have been standardized, there are some areas in which equipment manufacturers have a number of technological choices which will meet the relevant standards, and other areas in which standardization has not been enforced. An area which is not highly standardized is that of Operation and Maintenance (O & M) messages. Thus in order to access 0 & M messages a specific, proprietary format which is unique to an equipment manufacturer may be required. Secondly, due to the highly sensitive nature of some information stored within the fixed part of a network, a network operator severely restricts access to databases associated with network switching elements such as BSCs and MSCs. Finally, there is no guarantee that the information stored within databases associated with switching elements is up to date, since changes made to the system may not have been incorporated into these databases.
It is known to carry out independent monitoring of the signalling network of a cellular mobile radio network, for example by temporarily or permanently installing monitor probes on one or more links to monitor messages flowing on the link or links. A number of techniques are known for monitoring signalling messages in order to determine various parameters of a cellular mobile radio system. For example, European patent application 0 710 043 discloses a technique for determining the location area information for cells within a GSM-type network, and European application 0 658 032 (U.S. Pat. No. 5,521,902) discloses a technique for identifying the A interface link on which a monitor probe is installed.
Co-pending US patent application with U.S. patent application Ser. No. 08/988,466, which is commonly owned with the present application and which claims priority from European patent application 96 3 08 986.7 published as European Publication No. 0848567 on Jun. 17, 1998 (filed on the same date as the priority application of the present application), is hereby incorporated by reference. This co-pending application discloses a network discovery method for identifying radio elements within a cellular mobile radio network.